1. Field of Invention
This invention describes a one step synthesis of nanocrystalline zeolites having controlled particle size and micro and meso porosity.
2. Background
High surface areas and ion exchange properties of zeolites, as well as tunable silicon to aluminum ratio (SAR) are desirable properties which lead to their use as catalysts for a variety of different reactions. One of the most commonly used zeolites is ZSM-5, a zeolite with MFI type framework, 0.54 nm pore diameter, and SAR that can be varied from 10 to several hundred. ZSM-5 zeolite is usually synthesized with sodium cations acting as counter ions for the negatively charged framework. Sodium ions can then be replaced with other cations capable of entering the pores during a post-synthesis modification of the zeolite.
ZSM-5 zeolite has traditionally been used in the petroleum industry for fluid catalytic cracking due to the presence of acidic sites on its surface [1-4]. The ZSM-5 zeolite can be used in its H-form [1,2] or after ion exchange with different metal ions, such as nickel [3] or zinc [4]. Pt- and Ir-exchanged ZSM-5 zeolites are used in catalytic hydroisomerization of n-alkanes [5].
In the catalysis community, there has been a great deal of interest in nanocrystalline zeolites due to potential improvements in catalytic activity resulting from increased surface areas and decreased diffusion path lengths [6,7]. Several groups have compared the catalytic activity of nanocrystalline ZSM-5 to conventional micron-sized crystals [8-10]. Nanocrystalline ZSM-5 zeolites with particle sizes ranging from 20 to 50 nm were compared to microcrystalline ZSM-5 with respect to catalytic activity in epoxide rearrangement reactions [8]. The authors found that the nanocrystalline ZSM-5 zeolites had a significantly higher conversion rate and selectivity towards the desired products than the micron-sized zeolite. Choi and coworkers showed that micropore-mesopore composite materials formed from ultrathin sheets of ZSM-5 exhibit improved catalytic activity for methanol-to gasoline conversion [9]. These composite materials were also more resistant to deactivation occurring due to coke deposition [9,11]. Firoozi et al. showed that the use of nanocrystalline ZSM-5 resulted in a higher methanol to propylene conversion rate than for large ZSM-5 crystals [10].
Since the ZSM-5 zeolite pore diameter is relatively small, diffusion of products and reactants in the case of large micron-sized crystals is slow. Different approaches have been taken to improve the catalytic performance of ZSM-5 zeolites. One approach is treating large zeolite crystals with acids or bases in order to create mesopores by leaching the framework atoms. Zeolite crystals several hundred nanometers in size were treated with sodium aluminate followed by acid treatment in order to create mesopores 4 to 15 nm in diameter [12]. Ogura used an aqueous sodium hydroxide solution in order to create mesopores in ZSM-5 crystals and concluded that the mesopore volume progressively increased with longer base treatment times [13]. However, the leaching technique can cause a change in the Si/Al ratio of the zeolite via selective dealumination or desilylation depending on the nature of leaching agent [12,13].
Nanocrystalline zeolite samples have been synthesized from reaction mixture gels [14-16]. The effect of crystallization time on the zeolite crystal size was studied by Mohamed et al. [14]. The reaction mixture was composed of sodium aluminate, fumed silica and tetra-n-propylammonium hydroxide (TPAOH), and hydrothermal treatment at 230° C. produced 55 to 80 nm sized crystals. The crystal size decreased at longer synthesis time, while the crystallinity increased. Song and coworkers found that the removal of ethyl alcohol in the reaction mixture produced during hydrolysis of TEOS (tetraethylorthosilicate), a source of silicon, affects the crystal size of the zeolite nanocrystals [15]. Majano and coworkers employed ZSM-5 seed crystals in a template free reaction mixture in order to obtain ZSM-5 particles 30-70 nm in size [16]. Larger zeolite crystals were obtained when the reaction temperature was increased from 100-120° C. to 170° C.
Several reports describe the synthesis of ZSM-5 zeolites with both micro- and mesopores [9,17-19]. Choi and coworkers used a bifunctional surfactant, composed of a 22 carbon atom alkyl chain and two quaternary ammonium groups separated by a C6 alkyl linkage. The surfactant formed micelles, in which the quaternary ammonium atoms were located in planes, where the crystallization of ZSM-5 nanosheets occurred [9]. Zhu and coworkers employed a double template system, where the TPAOH template was governing crystallization of the MFI zeolite phase and polyvinyl butyral was used as a mesopore directing agent [17]. Xin et al. used a combination of TPABr (tetra-n-propylammonium bromide) and [3-(trimethoxysilyl)propyl]octadecyl-dimethylammonium chloride to synthesize iron-exchanged ZSM-5 powders containing mesoporous aggregates of smaller than 50 nm microporous ZSM-5 particles and tested the catalytic activity of the zeolite in selective hydroxylation of benzene to phenol [18]. Li and coworkers used a TPAOH/L-lysine co-template system and a two-step synthesis to prepare microspheres composed of stacked nanocrystals approximately 35 nm in size [19]. A micro-mesoporous composite with zeolite crystals around 20 nm in size was synthesized by using a mixture of TPAOH and alkyltriethoxysilane [20]. Chmelka and coworkers used a three step double template system, where ZSM-5 was precrystallized with TPAOH template followed by addition of mesopore forming phenylaminopropyltrimethoxysilane and heated at 90° C. for 6 hours. The resulting reaction mixture was then hydrothermally treated at 170° C. for 7 days, and zeolite crystals 5-10 nm in size were produced [21]. Fang and coworkers reported that the synthesis of mesoporous ZSM-12 zeolite is possible without the use of mesopore directing agent [22]. 20-30 nm zeolite nanocrystals forming aggregates under 1 μm in size were synthesized from a supersaturated reaction mixture. The authors concluded that formation of mesoporous aggregates using a single template system is achievable by a careful choice of the zeolite nucleation conditions, and that the zeolite nanocrystals form mesopores through self assembly. Tao et al. summarized different methods of mesoporous zeolite synthesis in their review work [23].
Zeolite beta is another commonly used zeolite in a number of catalytic reactions. Zeolite beta possesses intersecting pore channels approximately 6.6 Å in diameter, which places them between ZSM-5 (5.6 Å pores) and faujasite (7.4 pores) zeolites. The silicon to aluminum ratio in zeolite beta has been varied greatly, and lower Si/Al ratios have been found to favor a faster crystallization rate [30]. Pt and Pd loaded zeolites have been used for the selective hydrogenation of toluene with complete conversion and 100% selectivity under optimal conditions [31]. Modhera and coworkers studied hydroisomerization of 1-hexene over platinum loaded nanocrystalline zeolite-beta [32]. Nie et al. investigated the conversion of citral to menthol by Zr-loaded nanocrystalline zeolite beta. The use of zeolite beta resulted in a high yield as well as high diastereoselectivity towards the desired product [33]. Ding and coworkers reported that nanocrystalline zeolite beta with W—Ni catalyst had higher hydrodesulfurization, hydrodearomatization and hydrodenitrogenation activities than conventional micron-sized zeolite particles [34].
Different approaches to synthesis of hierarchical zeolite materials were summarized in a review by Pérez-Ramírez and coworkers [35]. Soler-Illia et al. discussed the fundamentals of zeolite synthesis and production of hierarchical zeolites in [36]. Mesoporous zeolites have been prepared by post-synthesis treatment of zeolites, such as leaching of large crystals with acids, [37] as well as inorganic [38] and organic bases [39-41]. During such treatments, selective desilylation or dealumination of the zeolite occurs leading to formation of mesopores. Pérez-Ramírez and coworkers employed a partial detemplation and disililation technique to more precisely control mesoporosity in b zeolite [41]. Chou and coworkers found that a post-synthesis treatment of zeolites with a strong base decreases the crystallinity and micropore volume of the material [42]. A method described by Mohr and Janssen described synthesis of mesoporous zeolites by synthesizing zeolite crystals first, then binding them with silica material and converting the silica binder into zeolite during the subsequent hydrothermal treatment [43].
Multiple approaches have been used to synthesize zeolite beta based hierarchical materials [44-50]. Camblor and coworkers studied the effect of Si/Al ratio on the crystal size of zeolite beta and found that crystals under 15 nm in size could be synthesized at low Si/Al ratios [44]. The mesopores originate from the interparticle distance, which are non-aggregated in the case of as-synthesized material and may be partially sintered after calcination. Kuechl and coworkers studied synthesis of pure silica zeolite-beta nanoparticles under 100 nm in size and different degree of aggregation using 4,40-trimethylenebis(1-methylpiperidine) template [45]. Several studies involved using a “hard template” approach, when a porous material, such as carbon black or silica gel is used to assist in mesopore formation [46-48]. Tong and coworkers employed a silica monolith and converted the amorphous walls into sodium beta zeolite, while using carbon as a transitional template and meso/macropore formation agent [46]. Valtchev et al. used a silica containing vegetal template (Equisetum arvense leaves and stems) in order to synthesize a hierarchical zeolite material containing micro, meso and macropores [47]. Lei and coworkers synthesized micromesoporous zeolite beta by modifying a bimodal pore silica gel with zeolite beta seed crystals and hydrothermally treating the system [48].
A “soft template” approach typically involves two molecular templates used simultaneously or in sequence to induce formation of the zeolite beta as a microporous phase with mesoporous areas between zeolite crystals [49-51]. Bagshaw and coworkers synthesized a zeolite beta/mesoporous silica composite which contained a structured mesopore and microporous zeolite areas [49]. Composite materials using MCM-41/zeolite-beta composites were synthesized by Xu et al. via dissolution of zeolite beta particles and subsequent hydrothermal treatment with hexadecyltrimethylammoniumbromide as a mesopore directing agent [50].
Aguado and coworkers prepared a hierarchical zeolite-beta material by hydrothermally treating zeolite beta nanocrystals, whose surface was functionalized with organosilanes. The organic moieties provided mesopore spacing between the zeolite crystals. The textural properties of the composite material depended on the type and concentration of organosilane groups on the zeolite seed crystals [51]. Fang and coworkers previously showed that synthesis of mesoporous MFI zeolite by adding a polyanion nucleation promoter H2PO4− is possible without the presence of a second, mesopore forming template [52]. Liu et al. demonstrated the synthesis of zeolite beta aggregates without a secondary template at different Si/Al ratios, with lowest primary particle size of 80 nm [53]. Different approaches to preparation of mesoporous zeolites have been summarized by Tao and coworkers [23].